Known fiber optic communications systems include a laser diode, a modulator and a photodetector diode. Modulators are either direct, modulating the optical wave as it is generated at the source, or external, modulating the optical wave after it has been generated. A problem with fiber optic communications systems is that the transmission distance is strongly dependent on the modulation fidelity. External modulation of lightwave signals is accomplished by adjusting a modulation chirp parameter to a substantially fixed value in a predetermined, controllable manner. This minimizes the transmission power penalty caused by chromatic dispersion in an optical fiber communication system.
External modulation is accomplished, for example, in a dual waveguide device wherein substantially identical input optical beams are supplied to the waveguides and wherein each waveguide is subject to its own individual, mutually exclusive control. Modulation signals are applied to each waveguide via the separate control. Moreover, control signals are applied to each waveguide for adjusting the modulation chirp parameter to a desired non-zero substantially fixed value.
An electro-optical modulator modulates the optical signal with an electromagnetic signal, preferably an RF signal. The RF signal interacts with the optical signal over a predetermined distance. The optical modulators slows the RF signal relative to the optical signal so that it takes the RF signal a longer period of time to travel the interaction distance. Therefore, the RF signal electric field, which modulates the optical signal, varies relative to the optical signal along the interaction distance. Since the RF signal does not act on the same portion of the optical signal throughout the interaction distance, the magnitude of modulation is reduced. The longer the interaction distance, the greater the reduction.
Typical high-speed electro-optical external modulators use a traveling-wave electrode structure. Such modulators have a microwave transmission line in the vicinity of the optical waveguide. A microwave signal and an optical signal co-propagate for a prescribed distance, thereby acquiring the required optical modulation. To prevent velocity mismatch between the microwave signal and the optical signal in a traveling wave modulator, a thick buffer layer is provided on a wafer to speed up the propagation of the microwave signal. Previously, a silicon dioxide (SiO.sub.2) buffer layer was created through known techniques such as electron beam, sputtering, or chemical vapor deposition (CVD). The buffer layer may be planarized throughout the wafer or may be patterned with electrode structures.
Using a SiO.sub.2 buffer layer has numerous disadvantages. Producing a SiO.sub.2 buffer layer requires expensive capital equipment and very precise control of production parameters. For example, devices such as evaporators, sputtering machines, gas supply machines or CVD machines cost tens or hundreds of thousands of dollars. Furthermore, most of the time, the SiO.sub.2 material has less oxygen than necessary and requires annealing to gain proper dielectric properties. During annealing, thermal expansion creates stress between the silicon dioxide layer and the optical waveguides. The waveguides can become non-uniformly stressed throughout the wafer and even disappear under certain conditions. In addition, SiO.sub.2 is a porous material, and absorbs a few percent of moisture after a 24-hour boil.
For many applications, it is important that the performance of electro-optical modulators be very stable over time and through temperature changes. Some electro-optic modulators are sensitive to temperature changes. For example, lithium niobate (LiNbO.sub.3) integrated optical devices made using Z-cut crystal orientation are particularly sensitive to temperature changes. The term Z-cut LiNbO.sub.3 refers to LiNbO.sub.3 that is cut perpendicular to the Z-crystallographic orientation. Such modulators are being used in high-speed telecommunications systems because they have relatively high modulation efficiency.
Z-cut LiNbO.sub.3 is sensitive to temperature changes because the pyroelectric effect in LiNbO.sub.3 creates mobile charge when temperature fluctuations occur in the substrate. The mobile charges can generate strong electric fields in Z-cut crystals during normal operation of the device. These electric fields are stronger in Z-cut than X-cut LiNbO.sub.3 crystals. Such strong electric fields are problematic because they can change the operating (bias) point of an electro-optic modulator, such as a Mach-Zehnder Interferometer (MZI), by creating fields across the waveguides that do not match one another. In addition, these strong electric fields can cause time dependent or uncontrolled charge dissipation, which may result in a loss of transmitted data. These fields may also cause arcing, which may also result in a loss of transmitted data.
There are methods known in the art for bleeding off pyroelectric charge. For example, some prior art devices use a metal oxide or semiconductor layer that is formed on top of the device to bleed off pyroelectric charge. Both amorphous and polycrystalline-silicon (poly-Si) semiconductor layers have been used to bleed off pyroelectric charge. A diffusion suppressing layer is sometimes included to prevent the metal electrodes from diffusing into the semiconductor bleed-off layer. Other prior art devices use a conductive layer on the bottom of the device that is electrically connected with the ground electrodes to provide a discharge path. In these devices, charge accumulating on the hot electrode can find a path to ground through the driver or biasing electronics.
A problem associated with LiNbO.sub.3 modulators is undesirable charge generation and charge redistribution that can occur when a bias voltage is applied to an electrical input of a LiNbO.sub.3 Mach-Zehnder interferometric modulator. The bias voltage is used to control the operating point of Mach-Zehnder interferometer. The application of the bias voltage can cause the formation of mobile charges, either in the form of electron, holes, or ions. These mobile charges either counteract the effect of the applied voltage by establishing a positive DC drift, or enhance the applied bias voltage by establishing a negative DC drift. Positive drift is particularly problematic because the voltage required to maintain the bias condition will steadily increase ("runs away") causing a reset to occur, which will result in a loss of data.
There are methods known in the art for reducing DC drift caused by undesirable charge generation and charge redistribution. For example, some prior art devices reduce DC drift by using a SiO.sub.2 buffer layer that includes at least one metal oxide. The introduction of metal oxide(s) in the buffer layer can enhance the long term negative DC drift, which offsets the undesirable positive DC drift.
The prior art techniques for reducing DC drift and for enhancing charge bleed off add significantly to the cost of manufacturing the device. For example, costly silicon dioxide (SiO.sub.2) deposition systems must be used for many prior art techniques because high quality SiO.sub.2 is required. The quality and composition of the SiO.sub.2 layer is critical to minimizing bias drift because impurities in the layer affect charge mobility. Prior art techniques carefully control the resistivity of the buffer layer to minimize DC drift.
It would be advantageous to provide a method of manufacturing optical devices that is less expensive, less complex, and yielding higher quality optical devices than prior art methods. It would also be advantageous to provide an inexpensive method of manufacturing optical devices that reduces pyroelectric effects and undesirable charge generation and redistribution that can occur when a bias voltage is applied.
Benzocyclobutene (BCB) exhibits several advantages over materials such as SiO.sub.2, which are conventionally used in integrated optical devices. BCB is a new class of organic dielectric materials commonly used in multichip module (MCM) technology. As a result of its common use in MCM applications, BCB is a well-known and well-understood material. BCB has lower dielectric loss, a lower dielectric constant, is subject to lower mechanical stress, and is much easier to process during production of integrated optical modulators. The simplicity of forming BCB buffer layers provides a significant advantage over conventional buffer materials. A liquid BCB solution is applied to a wafer cured in a nitrogen atmosphere and patterned with a photoresist or metal mask. No expensive deposition machines, such as CVD machines, are required.
Unfortunately, interface adhesion forces between BCB and thin metal film is poor, resulting in a weak bond between the BCB layer and the metal film layer in an optical device. Furthermore, a velocity matched modulator requires an extremely thin layer of BCB, which may be less than one micron. It would be advantageous to provide a method of manufacturing optical devices which uses BCB as a buffer layer. It would also be advantageous to provide a conductive BCB buffer layer that reduces pyroelectric effects and undesirable charge generation and redistribution that can occur when a bias voltage is applied.